1. Field of the Invention
The present invention generally relates to an electromagnetic clutch, and in particular, to an improved clutch rotor designed to reduce the amount of electromagnetic flux that leaks through the rotor and to an improved clutch rotor designed to uniformly distribute the driving forces across the radius of the rotor.
2. Description of the Prior Art
Electromagnetic clutches are well known in the prior art and may be used to control the transfer of power from an automobile engine to the refrigerant compressor of an automotive air conditioning system. The general structure of an electromagnetic clutch for an automobile air conditioning compressor is disclosed in U.S. Pat. Nos. 3,044,594 and 3,082,933, both of which are hereby incorporated by reference.
The construction of an electromagnetic clutch is shown in FIG. 1. The clutch assembly is disposed on the outer peripheral portion of annular tubular extension 2, which projects from an end surface of an unshown compressor housing to surround drive shaft 3. The clutch assembly includes rotor 5 rotatably mounted on tubular extension 2 by hearings 6. Rotor 5 is driven by a belt coupled to the automobile engine (not shown). As best seen in FIG. 2, axial end plate 5d' of rotor 5 is provided with a plurality of concentric rings 50', 52', 54', three of which are shown in FIG. 2. Concentric rings 50', 52', 54' are defined by a plurality of alternating arcuate slits 5a', 5b', 5c' and ribs 200', 202', 204'. For example, inner ring 50' is defined by a plurality of alternating arcuate slits 5a' and ribs 200'. Middle ring 52' is defined by a plurality of alternating arcuate slits 5b' and ribs 202'. Outer ring 54' is defined by a plurality of alternating arcuate slits 5c' and ribs 204'. Slits 5a', 5b', 5c' form magnetic poles on axial end plate 5d' of rotor 5.
Axial end plate 5d' of FIG. 2 forms part of six pole clutch. In this configuration, the three concentric rings 50', 52', 54' carry less magnetic flux than the remaining areas of the rotor. One magnetic pole face is defined by the annular area of the disc located radially inwardly of inner ring 50', two poles are defined by the annular area between inner ring 50' and middle ring 52', two additional poles are defined by the annular area between middle ring 52' and outer ring 54', and the sixth pole is defined by the annular area located outwardly of outer ring 54'.
Hub 7 (FIG. 1) is fixed to the outer terminal end of drive shaft 3 extending beyond tubular extension 2. Armature plate 8 is flexibly joined to hub 7 by a plurality of leaf springs 9. Leaf springs 9 are fixed to the outer surface of armature plate 8 by rivets 11. The axial end surface of armature plate 8 faces axial end plate 5d of rotor 5 with a predetermined axial air gap G therebetween. The axial end surface of armature plate 8 is provided with concentric arcuate slits 8a, 8b forming pole face 8c. Slits 8a are positioned to be opposite the midway point between slits 5a, 5b on axial end plate 5d while slits 8b are positioned to be opposite the midway point between slits 5b, 5c on axial end plate 5d.
Electromagnet 10 is mounted on compressor housing 1 concentric with drive shaft 3. Electromagnet 10 includes electromagnetic coil 101 disposed within annular hollow portion 5e of rotor 5 and is surrounded by an air gap. When coil 101 of electromagnet 10 is energized, pole face 8c is attracted to axial end plate 5d. Thus, drive shaft 3 rotates as rotor 5 is turned by the engine. If coil 101 of electromagnet 10 is not energized, pole face 8c of armature plate 8 is separated from axial end plate 5d by the recoil strength of leaf springs 9. Rotor 5 still rotates in response to the engine output, but drive shaft 3 is not turned.
Upon energization of electromagnetic coil 101, magnetic flux M, which is produced around electromagnet 10, passes through a magnetic passageway formed within electromagnet 10, rotor 5 and armature plate 8. Since magnetic flux M tends to follow the shortest path through the magnetic passageway, the flux M passes through rotor 5 and armature plate 8 in a zigzagging manner, as indicated by the dotted line in FIG. 1.
In the prior art axial end plate construction of FIG. 2, however, all of the ribs 200', 202', 204' are aligned along the same radii R'. Additionally, the angle .theta..sub.1 ' between ribs 200' on inner ring 50' is the same as the angle .theta..sub.2 ' between ribs 202' on middle ring 52', and the angle .theta..sub.3 ' between ribs 204' on outer ring 54' is the same as angles .theta..sub.1 ' and .theta..sub.2 '. Consequently, some of the electromagnetic flux "leaks" through the radially aligned ribs 200', 202', 204'. The leakage volume of the magnetic flux passing through radially aligned ribs 200', 202', 204' in a conventional electromagnetic clutch can be more than 25% of the entire magnetic flux generated by the electromagnet. As a result, the attractive force between the axial end plate and the armature plate is reduced. To compensate for this flux leakage, the number of coils in the electromagnet must be increased which in turn increases the size of the compressor.
In addition, rotor 5 of FIG. 1 is constructed with a plurality of V-notches for engagement with a plurality of belts. Consequently, a large bending moment due to the additional belts acts on the rotor. This bending moment is in turn transmitted to the ribs 200', 202', 204' as a bending stress represented by the following equations:
______________________________________ Bending stress: S = M / Z (1) Bending moment: M = W * l (2) Section Modulus: Z = (b * h.sup.2) / 6 (3) ______________________________________
where S is the bending stress, M is the bending moment, W is the load of the belt, 1 is the length of the rib from the position of load W, Z is the section modulus, b is the width of the rib, and h is the thickness of the frictional surface portion of the rotor.
For example, with reference to FIG. 3, there is shown a free body diagram of a rotor with potential relative radial positions of the ribs. If the load W of the belt is 10 kg, and the length 1 of the ribs 204', 202', 200' from the position of the load W are 1 cm, 2 cm, and 4 cm, respectively, the bending moment M experienced by rib 204' is 10 kg-cm, rib 202' is 20 kg-cm, and rib 200' is 40 kg-cm. Accordingly, applying the bending moment M to equation (1) above, the bending stress for each of ribs 204', 202', 200' are as follows: EQU S.sub.204' =10/Z EQU S.sub.202' =20/Z EQU S.sub.200' =40/Z
If the section moduli of the ribs are the same (i.e., the widths between the ribs are all equal, W.sub.1 '=W.sub.2 '=W.sub.3 '), the largest bending stresses occur at rib 200'. Thus, the ribs, if designed to evenly distribute the bending moment, should be the widest at the radially innermost position. Therefore, rib 200' should be the widest. However, all of the ribs on prior art rotors are virtually identical in size despite their relative radial positioning on the rotor. Consequently, the inner ribs are subject to more fatigue than the outer ribs.